Top quark measurements give 'God particle' new lease on life

Researchers from the University of Rochester have helped measure the elusive top quark with unparalleled precision, and the surprising results affect everything from the Higgs boson, nicknamed the "God particle," to the makeup of the dark matter that comprises 90 percent of the universe. The scientists developed a new method to analyze data from particle accelerator collisions at Fermilab National Accelerator Laboratory, which is far more accurate than previous methods and has the potential to change the dynamics of the Standard Model of particle physics. Details of the research are in today's issue of the journal Nature.

"This is a remarkable achievement in the measurement of the top quark," says Thomas Ferbel, professor of physics and astronomy at the University of Rochester, and a principal author of the paper. "The improvement has caused quite a stir because it has changed the accepted mass of the top quark in such a way that the Higgs boson is now in an energy range we have yet to explore. It's as if we've been digging a hole for the Higgs, and suddenly we realize we read the map wrong and it's really somewhere else." The masses of the top quark and Higgs boson are critical to understanding how the quantum world works, including answering one of science's great conundrums--what gives mass, mass?

The revision of the top quark mass started as a thesis project for one of Ferbel's doctoral students, Juan Estrada. He decided to see if there were a better way to calculate the mass of the top quark from the measurements already collected at Fermilab's particle accelerator. Ferbel was initially skeptical since scientists figured they'd wrung every bit of information from the data collected since the top quark's discovery in 1995. But Estrada, along with Fermilab scientist Gaston Gutierrez, developed a method based on probabilities that seemed to give a dramatic increase in precision. Ferbel brought in a third student, Florencia Canelli, to help extend the method to calculate the top quark's spin properties as well as its mass.

When the real-world data was parsed, the method yielded a nearly 40 percent increase in precision; less than predicted, but still a tremendous boon to physicists. The improved method allows researchers to glean as much information from the available data as would have been possible from a sample two and a half times as large, which is invaluable when collecting data from each collision is such an delicate and arduous task.

The second major fallout from the new measurements is that the Higgs boson--the particle that is theorized to give rise to mass itself--apparently exists at higher energy levels than where scientists have been searching. Since all subatomic particles are related to each other, changes in the characteristics of one ripples through other particles, and since the top quark is especially massive, changes to it result in the largest changes in other particles--especially the Higgs. Based on the old accepted value of the top quark mass, physicists expected to find the Higgs boson at around 96 GeV/c2 (gigaelectron-volts), but have been able to rule out that it actually exists there. That threw the whole Standard Model into a quandary. The new measurement for the top quark mass, however, now places the Higgs at about 117 GeV/c2, which is a range accelerators haven't yet searched, putting the elusive Higgs back into play.

"No matter how hard we try to break the Standard Model, it always seems to flex and still work," says Ferbel. "It's puzzling because we know in the long run the model isn't quite right, but it won't be beaten down. Every time we put stress on it, it shows it's still alive and breathing."

The new technique took a probabilistic approach to the measurements gleaned from the Fermilab collider. When the accelerator smashes a quark and an anti-quark together, a top quark and an anti-top quark are occasionally created. These quickly decay into other particle types, which themselves decay into yet more particles before the Fermilab detectors can begin to study them. This means the researchers have to work backward, looking at the third generation particles and inferring how they were made back in time, much like looking at a scattering of pool balls and deducing where they were three moves ago. Traditionally, researchers would assign a mass to the initial top and anti-top quarks and figure out what the decayed results should look like, then compare those results with what the detectors actually saw. The new technique works similarly, but assigns probabilities to a range of initial masses, giving more importance to the most accurate readings. The result, when played out over many collisions, is a measurement that's much more precise.

"Effectively increasing the data by two and a half times makes an impossible cause possible, if you're on the edge of discovering something like the Higgs," says Ferbel.

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Fermilab is a national laboratory in Ill., funded by the Office of Science of the U.S. Department of Energy, operated by Universities Research Association, Inc.

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